It is known to concentrate trace substances entrained in a gas by passing the gas stream through a U-shaped tube or pipe, i.e. a U-bend, generally containing an adsorbent, and to immerse the U-bend in a coolant-containing vessel from which the ends of the pipe emerge to connect the U-bend in a pipeline carrying the gas. This method and apparatus have been found to be advantageous for the analysis of traces of hydrocarbons and chlorinated hydrocarbons in gases such as air samples which are drawn for environmental and atmospheric monitoring or for processes control purposes. The analysis usually is carried out by gas chromatography.
Since the limits of detectors for certain trace substances are high, it frequently is necessary to work with sample volumes of a liter or more.
Such large sample volumes, however, cannot be directly fed to a gas chromatograph and thus it has been necessary in the past to carry out a variety of steps to reduce the sample volume and concentrate the trace substances before a sample is injected into the gas chromatograph.
One of the methods used heretofore involves the adsorption and condensation of the trace substances on solid surfaces such as graphite, carbon, silica gel, porous glass beads, organic polymers and like molecular sieve, mechanical or chemical adsorbents, at corresponding low temperatures.
In general, therefore, use could be made of an adsorbent filled tube which had its U-bend immersed in liquid nitrogen in a Dewar flask. A predetermined amount of air was drawn through this enrichment tube and the trace substances which collected on the adsorbent could then be driven off to form the sample for gas chromatographic measurements.
It is also known to bring an enrichment tube in a heat-insulated vessel to a temperature of -180.degree. to -190.degree. C., by spraying it with liquid nitrogen. The enrichment tube may also be made part of a heat exchanger system which is cooled by passing liquid nitrogen through the heat exchanger.
These approaches have, however, a variety of drawbacks. For example, in systems using Dewar flasks, it was generally necessary to manually or mechanically raise and lower the Dewar flask so as to immerse the enrichment tube therein. Cooling by direct spraying with liquid nitrogen is wasteful of the liquid nitrogen since only the latent heat of evaporation of the liquid nitrogen is utilized.